This invention relates to a method of subjecting a brittle material such as glass, ceramic and crystal material to precise grinding at a constant pressure. More particularly, the invention relates to a method and apparatus for grinding a brittle material used in optical equipment such as cameras, video devices and microscopes.
The term "brittle material" used in the present invention is defined as being a hard and brittle material, namely amorphous materials such as optical glass, quartz glass and amorphous silicon, crystal materials such as fluorite, silicon, KDP, KTP (KTiOPO.sub.4) and rock crystal and ceramic materials such as silicon carbide, alumina and zirconia. These materials generally have a fracture and toughness value (critical stress intensity factor) K.sub.IC of less than 10 meganewton/m.sup.3/2.
In a case where these brittle materials are subjected to grinding work, the material often is machined in a "brittle mode region" accompanied by brittle fracture, referred to as cracking or chipping, etc., below the machined surface. However, it is known that if the grinding work is performed upon setting a sufficiently small grinding cutting depth, even these brittle materials can be machined in a "ductile mode region" without the occurrence of cracking and chipping, as in the manner of such metal materials as iron and aluminum.
Whether the grinding work is performed in the "brittle mode region" or the "ductile mode region" is decided by depth of cut per abrasive grain of the grinding wheel used in grinding. The minimum depth of cut at which brittle fracture occurs when the depth of cut is gradually increased from zero is referred to as the "critical depth of cut". This takes on a value that is specific to the material.
In a case where a brittle material such as glass, ceramics or crystal is subjected to precise grinding work under constant pressure, generally use is made of a fine-grain grinding wheel of resin bond or the like exhibiting elasticity. A resin-body grinding wheel is formed by mixing powder of phenol resin, polyimide resin or the like with abrasive grains, applying pressure molding and then baking the result.
According to a manufacturing process for manufacturing a spherical lens by constant-pressure grinding using a conventional contoured grinding wheel having a spherical shape, a pressed blank that has been molded into the shape of the spherical lens is subjected to coarse grinding in one or two stages, after which precise grinding referred to as fine grinding is applied. Finally, polishing by free abrasive grains is performed one or two times to finish the spherical shape. The resin-bonded grinding wheel generally is used as the grinding tool at the time of finishing before polishing referred to as fine grinding.
A precision grinding method with a fixed depth of cut referred to as "ductile-mode grinding" has been investigated in recent years at a number of research facilities. According to this method, the heights of the tips of abrasive grains in a grinding wheel are made uniform by high-precision truing, and use is made of a highly precise, highly rigid machine to mechanically apply a minute depth of cut that is less than the critical depth of cut of the ground material (where the critical depth of cut is that at which the removal of the ground material undergoes a transition from the ductile mode to the brittle mode when the depth of cut applied to the material is gradually increased). It has been clarified that as a result of this method, even brittle materials such as glass can be subjected to grinding work in the ductile mode region in the same way as metals. Further, the specifications of Japanese Patent Application Laid-Open (KOKAI) Nos. 5-16070 and 5-185372 give a detailed disclosure of techniques for grinding in the ductile mode region by truing in which the heights of the tips of the abrasive grains of a grinding wheel are made uniform in a highly precise manner.
However, this conventional method of grinding involves certain problems. Specifically, in a case where grinding is carried out using an elastic bonded grinding wheel such as the resin-bonded grinding wheel, a large number of the fine abrasive grains sink into the bond(material) owing to the elasticity exhibited by the bond itself. The removal of the ground material progresses in small increments by abrasive grains which cut into the ground material and abrasive grains which engage with projections on the surface of the ground material.
More specifically, in the sectional view of FIG. 11 schematically illustrating the state of fine grinding work performed using a resin-bonded grinding wheel 1, abrasive grains 3 are contained in a bond 2 in a sunken state. Since the tips of the exposed abrasive grains 3 are uniform in height to a certain degree, the cutting depths of the individual particles also are substantially uniform. By suitably selecting abrasive grain diameter as well as the elasticity of the bond, the cutting depths of all of the abrasive grains can be made less than critical depth of cut d.sub.c, and there are cases in which fine grinding in the above-mentioned ductile mode region can be performed in apparent terms. However, when such a resin-bonded grinding wheel is used, the depth of cut of the abrasive grains differs slightly from particle to particle owing to a difference in the sharpness of the grinding tips of the individual abrasive grains and a difference in the amount of cutting performed by each individual abrasive grain. As a result, there are instances where deeply cutting abrasive grains exceed the critical depth of cut d.sub.c, thereby causing a crack K, namely brittle fracture, in a ground material or workpiece 4. The end result is that stable grinding in the ductile mode region cannot be carried out. Further, when grinding proceeds and the surface of the ground material 4 takes on a high degree of flatness in highly precise constant-pressure grinding carried out by a resin-bonded grinding wheel, the abrasive grains 3 are engaged less often so that there is a gradual increase in the abrasive grains that do not participate in the removal of material. Consequently, even if machining time is prolonged, the amount of material removal diminishes to 7 or 8 microns and removal of material in excess of this figure cannot be performed.
Thus, precise grinding carried out by an elastic resin-bonded grinding wheel involves a number of unstable elements and is impractical since a great deal of know-how is required.
In grinding in the ductile mode region mentioned above, a minute depth of cut is set using a highly rigid, high-precision special-purpose machine that relies upon a grinding wheel in which the heights of the tips of the abrasive grains are rendered uniform by high-precision truing. This grinding method allows the machining of brittle materials such as glass in the ductile mode region.
FIG. 12 is a sectional view schematically illustrating the state of machining in ductile mode machining. Here the abrasive grains have been subjected to truing so that the exposed tips thereof have been worked to have a flat shape. In order to make the abrasive grains 3 of the grinding wheel cut into the workpiece 4 accurately by a depth of cut d as illustrated, grinding is carried out by applying a high load and performing positional control in such a manner that cutting will fall within the critical depth of cut d.sub.c, which is the limit within which the workpiece 4 will not sustain brittle fracture. In other words, this method of grinding in the ductile mode region requires that the depth of cut d be controlled and set in a highly precise manner. Since the highly rigid special-purpose grinding machine and an accompanying control unit must be prepared for this purpose, the cost of machining becomes very high.